WASHINGTON -- By inserting a "memory" into a plastic material, researchers say they have created a surgical thread that can tie itself into a knot. The same substance may also be used for medical implants such as stents or bone screws.

The material, made of thermoplastic polymers that can be absorbed by the body, can be engineered to assume a string-like shape at room temperature and then transform into a medically useful shape when warmed by body temperature, said Robert Langer, co-author of a study appearing Friday in the electronic version of the journal Science.

Langer, a professor of chemistry at the Massachusetts Institute of Technology, said the plastic could be used to make implants or bone screws that are not much bigger than a piece of string when inserted into the body. Once they warm up, the devices change to form the appropriate implant.

"In a test on mice, we showed we can make these sutures actually tie themselves," said Langer.

He said that since the material has a memory, it could be threaded into an incision as a loose knot. When it warms to the body's temperature, the material remembers its designed shape and size and shrinks to tighten the wound. Later, after the wound is healed, the material dissolves and is harmlessly absorbed by the body.

"It is like a smart suture," said Langer. "That could be very important in closing an incision in a place that is hard to reach by surgeon."

Langer said that modern, minimally invasive surgery often involves a small incision through which is passed a flexible tube. The surgical work is done by passing tools, stitching materials and even implants down the small diameter of the tube to site within the body.

By using materials that are small when they enter the body and then spring into the proper size later, it may be possible to do more complicated operations using the flexible tube technique, called laparoscopy, he said.

Dr. Frederick Finelli, head of laparoscopic surgery at the Washington Hospital Center, said the idea of having plastic implants with a memory was fascinating and potentially important.

"There are all kinds of possibilities in surgery for this type of material," he said.

More research on animals must be conducted before the material can be tested in humans, but Langer said the basic polymer materials have been used in other implanted medical devices and shown to be nontoxic.